A flexible electrical wiring board (which also refers to an electrical wiring film) link between two sections connected by a hinge in a cell phone and the like. In the hinge portion, the electrical wiring film is disposed so as to wound a hinge axis and the like with a curvature radius according to the size of the hinge.
However, the electrical wiring film has been recently complicated because torsional rotation and the like are adopted in the rotation mechanism of the hinge. That is, since the electrical wiring film is bent under more severe conditions, there has been a problem that the conventional electrical wiring is damaged because the conventional electrical wiring can not endure the stress generated at the time of bending.
To cope with this problem, a method of providing a slit penetrating a film at a stress concentrated part of an electrical wiring film has been disclosed in Patent Document 1. However, through the method, the slit is required to be provided in an extremely narrow space (for example, 0.1 mm or less) between the electrical wirings, therefore the slit is difficult to process.
In addition to this, a method of forming an electrical wiring film in a complicated shape is also proposed. However, through this method the area of the film becomes lager so that the cost increases and accommodating the film in an electronic apparatus and the like is difficult.
By the way, if electric transmission is replaced by optical transmission, the transmission rate per one channel may be increased, thus the number of transmission channels can be reduced. As a result, the width of the wiring film may be dramatically reduced so as to reduce of the stress generated on the film even if film is bent under severe conditions.
An optical waveguide is known as a member that performs optical transmission. In the optical waveguide, an inorganic material such as quartz glass, multicomponent glass or the like is used, which has characteristics of having a low optical propagation loss and a wide transmission band. Recently, optical waveguide made of a polymer material having excellent processability and cost performance attracts attention. As the polymer optical waveguide film, there has been proposed a flat optical waveguide composed of a core-clad structure in which a core is made of a polymer material excellent in transparency such as polymethylmethacrylate (PMMA) or polystyrene and a clad is made of a polymer material which has a lower refractive index than the core material. In addition, there has been proposed a flat optical waveguide with a low optical propagation loss using a polyimide which is a polymer material excellent in heat resistance and transparency (Patent Document 2). Since these optical waveguide films have flexibility, they are expected as a material in place of an electrical wiring film.
In the case where an optical waveguide is used for an electronic apparatus, an electrical wiring is also required for supplying electric power and the like in many cases, thus both of an optical wiring and an electrical wiring are required. In this case, it is preferable to use a flexible electrical and optical hybrid substrate (which also refers to an electrical and optical hybrid film) in which an optical waveguide and an electrical wiring board are integrally formed, which result in saving space, reduction of thickness and size, and improvement of the embedding workability.
However, even when an electrical and optical hybrid film is used, the number of the electrical wiring is increased in proportion to the sophistication of an electronic apparatus. When the number of the electrical wiring is increased, width of the film is also increased, which results in that the film becomes unable to endure severe bending. To solve this problem, a slit on an electrical wiring film may be provided or the shape of the film may be made complicated. However these methods can not solve the problem as described above.
That is, an electrical and optical hybrid film having a high bending resistance has been desired, which is not damaged even if it is bent under severe conditions, despite a simple shape such as a linear shape and the like.
Patent Document 1: Japanese Patent Application Laid-Open No. 2005-57259
Patent Document 2: Japanese Patent No. 2813713